A glow discharge starter is usually connected across or in parallel with an arc discharge lamp and contains a pair of electrodes. At least one of the electrodes comprises a bimetallic element which, when heated as a result of the glow discharge, bends towards the other electrode When contact is made, the glow discharge ceases causing the bimetallic element to cool and withdraw from the contacted electrode. When contact is broken, a voltage pulse induced by the induction of the ballast, appears across the opposed electrodes of the lamp thereby initiating an arc discharge within the lamp. If the lamp ignition does not occur after the first voltage pulse, the glow discharge sequence is repeated until lamp ignition occurs.
It is known to include a mixture of materials, which may comprise barium, magnesium and thorium, within the glow discharge starter. This mixture, although referred to a getter material or getter mixture, not only removes deleterious gases that may form during processing or during operation of the glow discharge starter, but also lowers the breakdown voltage of the starter. The getter material may be supported by a getter holder which consists of a small piece of metal in which a cup is generally formed. The getter mixture is contained within the cup. During fabrication and processing of the glow discharge starter, the getter mixture contained within the cup of the getter holder is "flashed" onto the internal surface of the envelope and internal parts of the glow discharge starter. Flashing is a well known process accomplished by means of a radio frequency generator commonly referred to as a bomber. The above mentioned process creates a more effective surface for improved gettering of deleterious gases within the glow discharge starter. However, to be effective at lowering the breakdown voltage, the material must be disposed on the electrically connected active parts of the starter.
The glow discharge starter is designed such that the contacts close at a voltage chosen between the maximum lamp voltage and the minimum supply voltage (i.e., closure voltage). The contacts of the starter must also remain open at voltages less than the maximum lamp voltage (i.e., non-reclosure voltage). The development of compact fluorescent lamps, wherein the glow discharge starter is contained within the lamp base, has placed more stringent requirements on the glow starters. One of these is the requirement for reliability in a high temperature environment up to about 200 degrees Celsius. Since a glow discharge starter is a temperature-sensitive device, the increased temperature tends to change the operating characteristics of the starter by decreasing the discharge gap between the free end of the bimetallic element and the counter electrode. Some of these high temperature glow discharge starters are also required to operate with higher wattage lamps (e.g., up to 50 watts). Among newly developed are 18, 22 and 28 watt compact fluorescent lamps. To be suitable to operate these three lamps, a starter should have a minimum closure voltage of 105 volts and a maximum non-reclosure voltage of at least 85 volts. It is important that the electrical parameters of the glow discharge starter remain within this range throughout the life of the starter. A conventional glow discharge starter intended for low lamp voltage applications does not meet the temperature requirement. Temperatures above 100-120 degrees Celsius generally disable these starters. Maintaining electrical parameters within the 105/85 volt range is difficult to control.
The switching transient voltage output of the device depends upon the flexure and shape of the bimetallic element. Greater flexure distortion normally causes higher pulse voltages. During this thermal distortion, the spacing between the bimetallic element and counter electrode is decreased and adversely affects the breakdown voltage. Keeping the breakdown voltage in the desired range, requires a larger gap. This inconsistency demands compromise and often means difficulties in production and increases in cost.
A solution to improve high temperature operation is to increase the spacing between the free end of the bimetallic element and the counter electrode. However, this solution often results in the loss of operating voltage control. For example, in a single discharge gap starter, increasing this spacing to compensate for the increase in ambient temperature, also increases the closure voltage of the starter. For high line voltage applications (i.e., 220-240 volts AC), the problem can be overcome with tight control of this spacing. However this can result in a smaller yield in production or higher cost.
Attempts have been made to avoid the above-mentioned problems by utilizing complex gases to stabilize the characteristics of the glow discharge starter during its life. These gas compositions have included light gases (e.g., helium and hydrogen) which can be absorbed by the starter envelope, getter or internal metal parts.
U.S. Pat. Nos. 4,845,406 and 4,938,727, which are assigned to the Assignee of the present Application, teach a glow discharge starter having a pair of discharge gaps. One discharge gap is formed between the curved portion of a bimetallic element and and a getter holder which is secured to a counter electrode. Another discharge gap is formed between the free end of the bimetallic element and the counter electrode. While the above glow discharge starter performs satisfactorily, this starter requires the use of a separate getter holder in order to form one of the discharge gaps.